This invention relates in general to drive train assemblies for transferring rotational power in vehicles. In particular, this invention relates to an improved method of manufacturing a drive line assembly for transmitting rotational power from an engine to an axle assembly in a vehicle.
In most land vehicles in use today, a drive train assembly is provided for transmitting rotational power from an output shaft of an engine/transmission assembly to an input shaft of an axle assembly so as to rotatably drive the wheels of the vehicle. To accomplish this, a typical vehicular drive train assembly includes a hollow cylindrical driveshaft tube. A first universal joint is connected between the output shaft of the engine/transmission assembly and the driveshaft tube, while a second universal joint is connected between the driveshaft tube and the input shaft of the axle assembly. The universal joints provide a rotational driving connection from the output shaft of the engine/transmission assembly through the driveshaft tube to the input shaft of the axle assembly, while accommodating a limited amount of misalignment between the rotational axes of these three shafts.
It is known that when any mechanical body is rotated about an axis, a natural resonant frequency is defined thereby. This natural resonant frequency is an inherent characteristic of the mechanical body and is based upon many factors, including its composition, size, and shape. When the mechanical body is rotated at a speed which is at or near its natural resonant frequency, a relatively large amount of vibration can occur. In the context of a vehicular driveshaft tube, the natural resonant frequency is often referred to as the "critical speed" thereof. Thus, when a driveshaft tube is rotated at or near its critical speed, it can begin to vibrate excessively. Such vibration can, at a minimum, generate undesirable noise in the vehicle during operation. More seriously, this vibration can cause excessive stresses and rapid failure of not only the driveshaft tube, but the other components of the drive train assembly connected thereto. Accordingly, an important consideration in the design and manufacture of driveshaft tubes and other drive train assembly structures is that they not be operated at or near their critical speeds in normal operation.
Thus, the critical speed for a driveshaft tube is a function of, among other things, the nature of the material used to form the driveshaft tube. Generally speaking, the lighter the material used to form the driveshaft, the lower the critical speed. Traditionally, vehicular driveshaft tubes have been formed from steel alloys. Steel alloys have been found to be acceptable materials not only because they possesses sufficient strength to transmit the rotational loads which are normally encountered during use, but also because they are relatively heavy and stiff materials. As a result, the critical speed of steel alloy driveshaft tubes is usually sufficiently high that it is not encountered during normal operation of the vehicle. Unfortunately, because they are relatively heavy materials, steel alloys contribute an undesirable amount to the overall weight of the vehicle. To address this, driveshaft tubes have more recently been formed from lighter weight alternative materials, such as aluminum alloys. Aluminum alloys have been found to be desirable for use in vehicular driveshaft tubes because they are much lighter than steel alloys, yet possess sufficient strength to transmit the rotational loads therethrough. Unfortunately, for this same reason of lighter weight, the critical speed of an aluminum alloy driveshaft tube is usually much lower than the critical speed of a comparably sized steel alloy driveshaft tube. The critical speeds of typical aluminum alloy driveshaft tubes have been found to be undesirably close to the normal operating speeds of the vehicle than comparable steel alloy driveshaft tubes.
As mentioned above, the critical speed for a driveshaft tube is also a function of the size and shape of the driveshaft tube. Generally speaking, the longer the driveshaft tube is in length and the smaller it is in diameter, the lower the critical speed. The use of aluminum alloys has allowed the replacement of traditional two-piece steel alloy driveshaft tubes with newer one-piece aluminum alloy driveshaft tubes. The newer one-piece driveshaft tubes are preferable to the traditional two-piece steel alloy driveshaft tubes because fewer parts are required. However, because they are longer in length, one-piece aluminum alloy driveshaft tubes have a lower critical speed than a comparably sized two-piece steel alloy driveshaft tubes, undesirably close to the normal operating speeds of the vehicle than comparable steel alloy driveshaft tubes.
Attempts have been made to alter the critical speed of one-piece aluminum alloy driveshaft tubes to facilitate their use in vehicles. For example, it is known that the critical speed of an aluminum alloy driveshaft can be increased by covering it with a coating of a high strength material, such as a resin matrix reinforced with graphite fibers. Though effective, the use of such a covering undesirably increases manufacturing costs. It would be advantageous, therefore, to provide an improved structure for a driveshaft tube which would enable the use of lighter weight aluminum alloys, yet would not require the use of relative expensive reinforcing coatings to raise the critical speed thereof above the normal operating speed of the vehicle.